1. Field of the Invention
The present invention relates to a novel class of crystalline microporous silicoaluminophosphates, to the method for their preparation, and to their use as adsorbents and catalysts. These compositions are prepared hydrothermally from gels containing reactive phosphorus, silicon and aluminum compounds and organic templating agents which function in part to determine the course of the crystallization mechanism and hence the structure of the crystalline product.
2. Description of the Prior Art
Molecular sieves of the crystalline aluminosilicate zeolite type are well known in the art and now comprise over 150 species of both naturally occurring and synthetic compositions. In general the crystalline zeolites are formed from corner-sharing AlO.sub.2 and SiO.sub.2 tetrahedra and characterized by having pore openings of uniform dimensions, having a significant ion-exchange capacity and being capable of reversibly desorbing an adsorbed phase which is dispersed throughout the internal voids of the crystal without displacing any atoms which make up the permanent crystal structure.
Other crystalline microporous compositions which are not zeolitic, i.e. do not contain AlO.sub.2.sup.- tetrahedra as essential framework constituents, but which exhibit the ion-exchange and/or adsorption characteristics of the zeolites are also known. Metal organosilicates which are said to possess ion-exchange properties, have uniform pores and are capable of reversibly adsorbing molecules having molecular diameters of about 6 A or less are reported in U.S. Pat. No. 3,941,871 issued Mar. 2, 1976 to Dwyer et al. Also a pure silica polymorph, silicalite, having molecular sieving properties and a neutral framework containing neither cations nor cation sites is disclosed in U.S. Pat. No. 4,061,724 issued Dec. 6, 1977 to R. W. Grose et al.
The most recently reported class of microporous compositions and the first framework oxide molecular sieves synthesized without silica, are the crystalline aluminophosphate compositions disclosed in U.S. Pat. No. 4,310,440 issued Jan. 12, 1982 to Wilson et al. These materials are formed from AlO.sub.2 and PO.sub.2 tetrahedra and have electrovalently neutral frameworks as in the case of silica polymorphs. Unlike the silica molecular sieve, silicalite, which is hydrophobic due to the absence of extra-structural cations, the aluminophosphate molecular sieves are moderately hydrophilic, apparently due to the difference in electronegativity between aluminum and phosphorus. Their intracrystalline pore volumes and pore diameters are comparable to those known for zeolites and silica molecular sieves.
Several years ago, when the synthesis of zeolitic aluminosilicates had become established as a significant field for research, a number of attempts by various investigators were made to isomorphously replace a portion of the SiO.sub.2 tetrahedra of zeolites with PO.sub.2 tetrahedra during the synthesis process. Barrer et al. (J. Chem. Soc. 1965, pgs. 6616-6628) attempted to synthesize the mineral viseite, which contains AlO.sub.2, SiO.sub.2 and PO.sub.2 tetrahedra, by hydrothermal crystallization from reaction mixtures containing silica, phosphorus and aluminum compounds along with the oxides of sodium and/or calcium. Although a number of aluminosilicates and phosphates were formed, no evidence of isomorphous substitution of phosphorus for silicon was found. Wacks et al. (U.S. Pat. No. 3,443,892, issued May 13, 1969) reported the preparation of a faujasite-type zeolite having the formula: EQU 0.5-1.1Na.sub.2 O:Al.sub.2 O.sub.3 :0-0.2P.sub.2 O.sub.5 : EQU 2.3-3.3SiO.sub.2 :0-7.2H.sub.2 O
It is not stated whether there was any isomorphous substitution of phosphorus into the zeolite lattice.
Substantial success in preparing zeolite analogues containing phosphorus was reported by Flanigen and Grose, Molecular Sieve Zeolites-I, ACS, Washington, D.C. (1971), using a synthesis technique utilizing gel crystallization involving controlled copolymerization and coprecipitation of all the framework component oxides, aluminate, silicate and phosphate into a relatively homogeneous gel phase, with subsequent crystallization at 80.degree. C. to 210.degree. C. This technique resulted in the production of aluminosilicophosphates with the following types of zeolite frameworks; analcime, chabazite, phillipsite- harmotome, Type A zeolite, Type L zeolite and Type B zeolite, all of which contained significant amounts of phosphorus (5-25 wt.-% P.sub.2 O.sub.5) incorporated into the crystal framework.
Insofar as is presently known, none of the heretofore synthesized phosphorus-containing zeolite analogues have been utilized commercially.
The substitution of phosphorus for silicon did not appear to impart any beneficial properties to the substituted compositions not possessed by their aluminosilicate analogues with the possible exception that the individual crystals tended to be significantly larger. To the contrary, many of the physical and chemical properties of the phosphorus-substituted analogues were inferior to those of the unsubstituted species. The substitution of phosphorus in the framework structure of a Type L zeolite resulted in approximately a 50% reduction in its adsorption capacity and also a reduction in apparent pore size from about 10 A to 6-7 A. Phosphorus-substituted Type B was not stable toward thermal activation, and several of the cation forms of other of the phosphorus-substituted compositions prepared by Flanigen and Grose were not as stable as their aluminosilicate analogues. There are no known reports on observed differences in catalytic activity resulting from any phosphorus substitution which may have been achieved by prior known techniques.